3 erasers cost $3.75. Which equation would help determine the cost of 10 erasers?
Answer: There are several equations that could help determine the cost, each with a slightly different approach. Let $x$ represent the unknown cost of 10 erasers. Since 10 erasers cost $x$ , we have the following proportion: $\dfrac{10}{x}$ We can write the fact that 3 erasers cost $3.75 as a proportion: $\dfrac{3}{\$3.75}$ The cost changes along with the number of erasers purchased, and so the two proportions are equivalent. $\dfrac{10}{x} = \dfrac{3}{\$3.75}$